1. Field of the Invention
The present invention relates to photoelectric converter devices and, more particularly, to a photoelectric converter device for accumulating carriers generated as a result of the incidence of light and for reading a signal based on the accumulated carriers which is used for solid-state camera devices, image input devices, facsimile machines, digital copiers, etc.
2. Description of the Related Art
Conventional photoelectric converter devices include, for example, that disclosed in Japanese Patent Laid-Open No. H6-292087. FIG. 4 is an equivalent circuit diagram of the photoelectric converter device disclosed in that publication. In FIG. 4, an emitter region 102 of an NPN bipolar transistor 101 is connected to a drain of an N channel MOS transistor 105. A source electrode 106 of the MOS transistor 105 is connected to ground or to a constant voltage source at a low potential. Turn on and turn off of the MOS transistor 105 is controlled by a signal .phi.R. The collector region 104 of transistor 101 is kept at a positive potential, while its base region 103 is always floating.
The operation of this photoelectric converter device will now be described.
First, an accumulating operation is started when the MOS transistor 105 is placed in a non-conducting state.
At this time, the emitter region 102 is in a floating state wherein it is initially at ground potential, and the base region 103 is set in a floating state wherein it is at a positive potential. Base region 103 provides the light receiving surface for the converter. When light enters in this state, carriers (holes) are accumulated in the base region 103 in an amount corresponding to the amount of the light. The potential of the emitter region 102 assumes a value which depends on the carriers accumulated in the base region 103.
A reading operation is performed using a method wherein the potential of the emitter region 102 is directly detected using an amplifier or the like or a method wherein a reading switch is connected to the emitter region 102 and the switch is placed in a conducting state to allow the potential to be read into an external load capacity. In the latter case, a part of the carriers accumulated in the base region 103 is lost.
Next, during an extinguishing operation, the MOS transistor 105 is placed in a conducting state. During the period when the MOS transistor 105 is kept in the conducting state, the holes accumulated in the base region 103, which are residual charge, are recombined with electrons injected from the emitter region 102 to the base region 103. When the MOS transistor 105 is placed in the non-conducting state thereafter, the extinguishing operation ends and the next accumulating operation begins.
A description will now be made on the effect of the residual charge in the base region 103 at the time when the extinguishing operation ends. This residual charge can give rise to a residual, image because it remains when the next accumulating operation is started. Therefore, the period during which the MOS transistor 105 is in the conducting state must be long enough to eliminate the residual charge sufficiently.
If the residual charge is too small, it is difficult to obtain a sufficient output in a low luminance region, which deteriorates the linearity of the photoelectric conversion characteristics. The reason is that the potential of the emitter region 102 of the bipolar transistor 101 is not increased unless the emitter-base junction is forward-biased to a certain degree. The smaller the capacity of the emitter-base junction, the smaller the amount of charge required to forward-bias the emitter-base junction. Therefore, by reducing the capacitance of the emitter-base junction, desirable linearity of the photoelectric conversion characteristics can be obtained in the low luminance region even if the residual charge is small.
As described above, it is possible to provide a photoelectric converter device in which the generation of residual images is suppressed and which exhibits photoelectric conversion characteristics having high linearity in a low luminance region by expanding the period during which the MOS transistor 105 is kept in a conducting state to a certain degree.
However, conventional photoelectric converter devices have a problem in that residual images become more significant when they are exposed to only a small amount of light. There is another problem in that in response to the incidence of light after a prolonged dark condition, they can not provide output at a level corresponding to the amount of the light.
The above-mentioned problems will be described in detail.
FIG. 5 is a configuration diagram illustrating the operation of one photoelectric conversion cell of a conventional photoelectric converter device.
In FIG. 5, an emitter region 102 of an NPN bipolar transistor 101 is connected to a signal line 108 through a reading switch 107. A load capacitor 109 is connected to the signal line 108, and a voltage Vout on the signal line 108 is output. The emitter region 102 can be fixed to a reference voltage (ground) through a MOS transistor 105. The signal line 108 can be connected to the reference voltage (ground) though a signal line reset switch 110.
FIG. 6 is a time chart illustrating the operation of one of the photoelectric conversion cells of the conventional photoelectric converter device. When a positive pulse .phi.S is applied to the reading switch 107, the reading switch 107 enters a conducting state; the base-emitter junction of the NPN bipolar transistor 101- is forward-biased; charges are accumulated in the load capacitor 109; and the voltage Vout is increased (reading operation).
Next, when a positive pulse .phi.R is applied to the MOS transistor 105, the MOS transistor 105 enters a conducting state; the emitter region 102 is fixed to the reference voltage (ground); and excess carriers in base region 103 are removed (extinguishing operation). When a positive pulse .phi.RE is applied to the signal line reset switch 110, that switch enters a conducting state, capacitor 109 is discharged through switch 110 and the voltage Vout becomes equal to the reference voltage (ground).
When the pulse .phi.R ends (the extinguishing operation ends), the next accumulating operation begins wherein carriers are accumulated in the base region 103 in accordance with the amount of the incident light, and the potentials of the base region 103 and the emitter region 102 in a floating state increase.
In order to examine the residual image characteristics of the conventional photoelectric converter device, the following measurement was made with the periods of the accumulating, reading, and extinguishing operations set at 5 msec., 1 .mu.sec, and 1 .mu.sec, respectively.
FIG. 7 is a time chart for measurement of residual images. The light source was periodically turned on/off in synchronism with the pulses .phi.S, and peak values of the voltage Vout were plotted. Actual measurements indicated by .smallcircle. deviated from ideal values indicated by .multidot.. In FIG. 7, VP represents the peak value of the voltage Vout that appeared when the light source was continuously on; VD represents the peak value of the voltage Vout that appeared when the light source was continuously off; VT represents the peak value of the voltage Vout that appeared during the reading operation after the first accumulating operation after the light source was turned on; and VZ represents the peak value of the voltage Vout during the reading operation after the first accumulating operation after the light source was turned off.
The result of the measurement of residual images is shown in FIG. 8 which indicates how the peak values VP, VT and VZ change with changes in the irradiated light amount when the light source is on.
The magnitude of the peak value VZ is determined by the amount of the excess majority carriers (holes) remaining in the P-type base region 103 after the extinguishing operation. If a high level of forward bias is applied to the base emitter junction (the irradiated light amount is large) during the extinguishing operation, the excess majority carriers (holes) rapidly extinguish as a result of the recombination, the same as for electrons injected from the N-type emitter region 102, and settle at a certain amount. If the level of the forward bias applied to the base-emitter junction is low (the irradiated light amount is small), fewer electrons are injected from the N-type emitter region 102. This makes the recombination in the P-type base region 103 slower and, therefore, suppresses the reduction of the excess majority carriers (holes). As a result, in FIG. 8, the peak value VZ becomes substantially flat after it increases while the irradiated light amount is small.
After repeated extinguishing operations with the light source in the off state, the level of the forward-bias applied to the base-emitter junction is very low. In the next accumulating operation, the light source is turned on, and majority carriers (holes) are accumulated in the P-type base region 103 in an amount corresponding to the amount of incident light. However, a part of the majority carriers (holes) is used for forward-biasing the base-emitter junction and therefore does not contribute to the output. As a result, in FIG. 8, the increase in the peak value VT is very small when the irradiated light amount is small and becomes substantially parallel with the peak value VP after a certain irradiated light amount is exceeded.
A residual image ratio RZ and a potential increase ratio RT are defined as follows.
RZ=(VZ-VD)/(VP-VD).times.100[%] PA1 RT=(VT-VD)/(VP-VD).times.100[%]
The curve "a" obtained by connecting the circles in FIG. 11 indicates the dependence of the residual image ratio RZ obtained from FIG. 8 on the irradiated light amount. The residual ratio RZ is great when the irradiated light amount is small. The curve "e" obtained by connecting the circles in FIG. 12 indicates the dependence of the potential increase ratio RT obtained from FIG. 8 on the irradiated light amount. The potential increase ratio RT is small when the irradiated light amount is small. Thus, the conventional photoelectric converter device has a problem in that its residual image characteristics are significantly deteriorated when used with a small irradiated light amount.
It is an object of the invention to provide a photoelectric converter device and a method for photoelectric conversion which solve the above-described problems.